This invention relates generally to metal-clad dielectric sheeting for use in manufacturing flexible electric circuitry, and more particularly to copper-clad polyimide having an improved resistance to undercutting from gold or tin plating baths. Metal-clad dielectric sheeting is especially useful in beam tape carrier technology for automated bonding of semiconductor chips to substrates and for flexible printed circuit boards. A typical beam tape carrier consists of a dielectric (such as polyimide) with metal-cladding (such as copper), the metal-cladding being etched to leave a series of free standing beams which are directly bonded to semiconductor chips. One of the preferred metallurgies for the bonding process requires that copper beams be gold or tin plated. However, gold and tin plating baths cause undercutting at the edges of the copper beams and sometimes complete delamination of the beams from the dielectric sheeting.
Various coatings and other treatments for copper foils have been proposed to prevent undercutting and delamination of the copper foil and the dielectric layer. Indeed, a three-layer tape product consists of copper foil bonded to polyimide with an epoxy or acrylic adhesive. The three-layer product has no undercutting when gold plated because there is no exposed copper-polyimide surface. Two-layer product has a number of advantages in flexible circuitry over three-layer product and among those are added flexibility, less weight, lack of contamination due to the adhesive materials, potentially better thermal stability due to the thermal breakdown of the adhesive layer, the ability to expose the copper through the dielectric by chemically etching the dielectric, and stability to radiation. The adhesive material also affects sonic transmission when transducers are part of the circuit.
U.S. Pat. No. 3,981,691 to Cuneo teaches a method of improving the bond between the copper and the dielectric and which also minimizes undercutting by hydrochloric acid on the copper-polyimide bond. In Cuneo, black chromium (chrome oxide containing at least 25 weight-percent oxygen) is electrodeposited on the metal layer, followed by casting of the dielectric layer onto the black chrome covered metal layer. The Cuneo patent indicates that the copperpolyimide bond is improved such that the metal conductors will not be delaminated from the polyimide later by exposure for five minutes in a 10-normal hydrochloric bath at 25 degrees Celsius. While the black chrome treatment improves resistance to undercutting over the polyimide cast directly onto copper, the black chrome layer does not entirely prevent undercutting and delamination. Furthermore, since black chrome cannot be electrodeposited on polyimide, the Cuneo method is only available to cast polyimide films; i.e. in lieu there of polyimide films that are produced by pouring liquid polyimide onto the metal layer. This limits the Cuneo method to copper on one side of the polyimide film and to flexible circuit applications having relatively thin layers of polyimide. Also, the Cuneo method cannot be used with biaxially oriented polyimide such as Du Pont's kapton (Kapton is the Du Pont trademark for their polyimide film) because the Kapton is precast and cannot be bonded to the metal without an adhesive. (Kapton is biaxially oriented during fabrication and thus has good dimensional stability and is available in thicknesses up to 5 mils.) Dimensionally stable polyimide films such as Kapton are much preferred in flexible printed circuit applications over the cast polyimide.
U.S. Pat. No. 3,781,596 to Galli, et al. suggests plating a layer of chromium or nickel onto the copper layer to promote adhesion to the polyimide layer that is subsequently cast or laminated onto the copper layer. This method has the same disadvantages as Cuneo in that the chromium or nickel is plated on the copper, not the polyimide, thus limiting the product to conductor on one side of the polyimide, and is also limited to cast polyimide films. Furthermore, Cuneo indicates at (column 1, lines 35-49) that the chromium or nickel layer of Galli does not prevent undercutting when the product is placed in a gold plating bath.
U.S. Pat. No. 3,729,814 to Wright, et al. describes a method for making hybrid circuit arrays using a thin flexible substrate such as polyimide, vapor depositing a layer of chromium or nichrome onto the substrate, followed by vapor depositing a gold layer on the chrome or nichrome layer. The gold layer is then etched to form resistors, conductors, chip mounting pads and interconnections. While Wright teaches a method of improving adhesion of gold to polyimide, Wright does not address the problem of undercutting the copper-polyimide boundary caused by gold and tin plating baths. Indeed, Wright is not concerned with electrodeposition at all, his method uses vapor deposition to form the layers.
Therefore, it is an object of the present invention to provide a metal-clad dielectric which resists delamination and undercutting of the metal when exposed to certain plating baths.
It is another object of the present invention to provide an adhesiveless flexible carrier product having thicker and dimensionally stable polyimide layers.
It is also an object of the present invention to provide an adhesiveness flexible carrier product having a metal layer on each side of the polyimide film.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.